In the present context the concept of "sorptive capacity" aims at the rate by which the absorbent material takes up liquids such as water or aqueous solutions, including body fluids such as urine, blood and menstrual fluids, or to the liquid-retaining capacity of the absorbent material, or to both of these characteristics. The sorptive mechanism may be adsorption or absorption, or a combination thereof.
In the present context the concept of "absorbent material" is intended to comprise, among other things, any paper or paper-like structure, that consists mainly of cellulosic fibres, the principal use of which is to absorb liquids such as water or aqueous solutions, including body fluids such as urine, blood and menstrual fluids. Exemplary of such papers are hygienic paper and paper to be impregnated with thermosetting resins for production of decorative laminates
As stated in Ullmann's encyclopedia of industrial chemistry, Vol. A 18, page 663, 1991, hereby incorporated by reference, the term hygienic paper encompasses cellulose wadding, soft tissue, and crepe paper, all useful within the household or sanitary fields. It is well known that the rate of liquid uptake and liquid-holding capacity of hygienic paper are important when used in these fields. This is particularly valid with regard to absorbent articles such as catamenial devices (e.g. sanitary napkins, pantiliners, tampons etc.), diapers, bandages, adult incontinence garments, and the like; good liquid uptake and liquid-holding capacity are obvious prerequisites for the function of such articles. However, when used, an article of this kind is constantly subjected to pressure imposed by the weight and the movements of the bearer, and thus it is important that the liquid-holding capacity is high enough to retain the absorbed liquid also under pressure. Furthermore, in order to give good comfort to the bearer, the article should provide a feeling of dryness, meaning that any rewetting from the article to the skin of the bearer should by avoided, raising the requirements with regard to liquid-holding capacity even higher. Conventionally, certain polymeric materials forming hydrogels in contact with water, known as "superabsorbents", have been utilised to enhance the sorptive capacity of such articles; however, although this capacity of the article is enhanced, as the liquid is bound to superabsorbent particles, the sorptive capacity of the cellulosic fibres making up the hygienic paper itself is not in fact enhanced by the use of such superabsorbents.
Another field in which the sorptive capacity of cellulosic material is important is that of laminates, particularly so-called decorative laminates, such as described in U.S. Pat. No. 3,373,071. Such laminates conventionally comprise at least three layers: a wear surface layer, a print or pattern layer beneath the wear surface layer, and a core layer supporting the wear surface and print layers. All layers consist of paper sheet impregnated with thermosetting resin. The wear surface layer usually consists of translucent paper impregnated with melamine resin. The impregnation is carried out by submerging the paper in an aqueous solution of the resin. Obviously the sorptive capacity of the paper is a critical parameter for the efficiency of the impregnation.
The problem to be solved by the present invention is thus to provide a method for production of absorbent material having improved sorptive capacity.